The present invention relates to a technique for driving a motor, and more particularly to a technique for driving a PWM-controlled motor.
FIG. 22 illustrates a configuration of a conventional motor driving device for driving a three-phase motor (having U phase, V phase and W phase) by energizing the motor coils with PWM control. The conventional motor driving device includes a power supply 1 for driving the motor, a Hall signal processing section 2, an energization switching section 3, a level shift section 4, an oscillation section 5, a comparison section 6, a PWM control section 7, three half-bridge circuits connected in parallel to one another, a current detection resistor R, and an amplifier A for amplifying a voltage across the current detection resistor R.
The Hall signal processing section 2 receives Hall signals from Hall elements to produce logical signals that indicate the rotor position. The energization switching section 3 receives the logical signals to determine phases to be energized. FIG. 23A to FIG. 23C each illustrate a phase current to be applied to an energized phase, which is determined by the energization switching section 3, wherein FIG. 23A illustrates a U-phase current, FIG. 23B illustrates a V-phase current, and FIG. 23C illustrates a W-phase current. FIG. 23D illustrates a torque command signal TQ for determining the current level of the phase current. In FIG. 23A, FIG. 23B and FIG. 23C, a portion above the time axis represents the application of the source current, and a portion below the time axis represents the application of the sink current. Note that in the example shown in FIG. 23A to FIG. 23D, since the level of the torque command signal TQ is constant, each phase current flows as a source current or a sink current at a constant current level.
In the conventional motor driving device, a motor coil is energized with a source current (or a sink current) at a current level according to the torque command signal TQ during a period of 120 electrical degrees. Then, the motor coil is not energized, and thus the current level is zero, during the following period of 60 electrical degrees. Then, the motor coil is energized similarly but with a sink current (or a source current). Three phase currents each having such a rectangular waveform are applied while being shifted from one another by 120 electrical degrees. As a result, at any given time, a total of two phases are being energized, one with a source current and the other with a sink current, while the remaining phase being not energized. Thus, in the conventional motor driving device, the energization switching section 3 determines a total of two phases to be the energized phases, one for the source current side and the other for the sink current side. Moreover, the energized phase determination is performed for every 60 electrical degrees.
The PWM control section 7 performs PWM control using a PWM control signal P for each of the energized phases determined by the energization switching section 3. When the PWM control signal P is turned ON, a transistor of a half-bridge circuit is latched ON through the energization switching section 3 and the level shift section 4, whereby a current flows from the power supply 1 to a motor coil. Then, when the PWM control signal P is turned OFF, the transistor of the half-bridge circuit is latched OFF, whereby the current flow from the power supply 1 is stopped. It is only required to control, with the PWM control signal P, either one of the source-current-side transistor and the sink-current-side transistor, and the other transistor that is not PWM-controlled can be fixed to ON. Herein, it is assumed that the source-current-side transistor is PWM-controlled, while the sink-current-side transistor is fixed to ON. Therefore, in FIG. 23A, FIG. 23B and FIG. 23C, each hatched portion is where PWM control is performed.
In the conventional motor driving device, there is an abrupt transition of the phase current at each phase current switching point, as illustrated in FIG. 23A to FIG. 23C. Such an abrupt phase current transition causes vibrations in the motor, thereby causing the motor to give noise.